Genetic analysis of differentiated functions related to the erythropoietic system was carried out using somatic cell hybridization techniques. Friend virus transformed murine erythroleukemia cells replicate as clonal lines in vitro and express many differentiated functions ofred cells. These cells were mated to mouse and human fibroblasts not expressing these differentiated functions and the resultant hybrids isolated and characterized. This represents the first determination of the level of gene regulation following extinction of differentiated functions in hybrid cells.